Definition and background:
Latin for short. At the begining of mensural notation in the 13th century, the brevis (or breve) was the indication for the short note value as opposed to the longa, the indication for long note value. See also breve.
For more information on the music term "Brevis" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary